The present invention is related to a process for making a work piece having a major phase of xcex1xe2x80x3 from a titanium alloy, and in particular a process for making a biocompatible low modulus high strength titanium-based medical device having a major phase of xcex1xe2x80x3.
Titanium and titanium alloys have been popularly used in many medical applications due to their light weight, excellent mechanical performance and corrosion resistance. The relatively low strength commercially pure titanium (c.p. Ti) is currently used as dental implant, crown and bridge, as well as denture framework. With a much higher strength than c.p. Ti, Ti-6Al-4V alloy has been widely used in a variety of stress-bearing orthopedic applications, such as hip prosthesis and artificial knee joint. Moreover, the lower elastic modulus allows the titanium alloy to more closely approximate the stiffness of bone for use in orthopedic devices compared to alternative stainless steel and cobalt-chrome alloys in orthopedic implants. Thus, devices formed from the titanium alloy produce less bone stress shielding and consequently interfere less with bone viability.
Various attempts at providing low modulus, high strength titanium alloys for making medical implants with less stress shielding have been proffered by the prior art. There is still a great interest in finding a lower modulus and higher strength titanium alloys. In addition, studies have reported that the release of Al and V ions from the medical implants might cause some long-term health problems, for example the low wear resistance of Ti-6Al-4V alloy could accelerate the release of such harmful ions.
A primary objective of the present invention is to provide a process for making a work piece, and in particular a biocompatible low modulus high strength medical device, from a titanium alloy free from potential harmful components.
Another objective of the present invention is to provide a process for making a work piece, and in particular a biocompatible low modulus high strength medical device, from a titanium alloy having a major phase of xcex1xe2x80x3.
In order to achieve the aforesaid objectives a process for making a work piece having an xcex1xe2x80x3 phase as a major phase from a titanium alloy according to the present invention comprises the following steps:
a) preparing a titanium alloy composition comprising at least one isomorphous beta stabilizing element selected from the group consisting of Mo, Nb, Ta and W; and the balance Ti, wherein said composition has a Mo equivalent value from about 6 to about 9;
b) fast cooling said composition having a temperature higher than 800xc2x0 C. to a temperature lower than 500xc2x0 C. at a cooling rate greater than 10xc2x0 C./second between 800-500xc2x0 C., so that the resulting cooled composition contains an xcex1xe2x80x3 phase as a major phase.
Preferably, said titanium alloy composition in step a) is substantially free from an eutectoid beta stabilizing element selected from the group consisting of Fe, Mn, Cr, Co, and Ni.
Preferably, said titanium alloy composition in step a) is substantially free from Al.
Preferably, said titanium alloy composition in step a) is substantially free from V.
Preferably, said titanium alloy composition in step a) consists essentially of at least one isomorphous beta stabilizing element selected from the group consisting of Mo, Nb, Ta and W; and the balance Ti.
Preferably, said cooling rate is greater than 20xc2x0 C./sec.
Preferably, said fast cooling in step b) comprises water quenching.
Preferably, said composition has a temperature of 800-1200xc2x0 C. before said fast cooling in step b).
Preferably, said preparing in step a) of the process of the present invention comprises casting said titanium alloy composition to form a work piece having a temperature higher than 800xc2x0 C., and said fast cooling in step b) comprises fast cooling said work piece having a temperature higher than 800xc2x0 C.
Preferably, said preparing in step a) of the process of the present invention comprises metal working said titanium alloy composition to form a work piece, and heating the resulting work piece to a temperature higher than 800xc2x0 C., and said fast cooling in step b) comprises fast cooling said work piece having a temperature higher than 800xc2x0 C.
Preferably, said titanium alloy composition in step a) further comprises one or more incidental impurities selected from the group consisting of carbon, oxygen and nitrogen, wherein a total amount of said one or more incidental impurities is less than 1 wt %.
Preferably, said work piece having a major phase of xcex1xe2x80x3 is a medical device.
The present invention provides a process for making a biocompatible low modulus high strength medical device from a titanium alloy, which comprises preparing a titanium alloy having a composition consisting essentially of at least one isomorphous beta stabilizing element selected from the group consisting of Mo, Nb, Ta and W; and the balance Ti, wherein said composition has a Mo equivalent value from about 6 to about 9; casting or metal working the titanium alloy to form a work piece; and quenching the work piece which is the resulting hot cast having a temperature higher than 800xc2x0 C. at a cooling rate greater than 10xc2x0 C. per second, or heating the work piece resulted from said metal working to a temperature higher than 800xc2x0 C. and quenching the work piece having a temperature higher than 800xc2x0 C. at a cooling rate greater than 10xc2x0 C. per second, so that the cooled work piece contains an xcex1xe2x80x3 phase as a major phase, and can be used as a medical device which is biocompatible, and has a low modulus and high strength.
In the present invention, said Mo equivalent value, [Mo]eq, can be represented by the following equation:
[Mo]eq=[Mo]+0.28[Nb]+0.22[Ta]+0.44[W]
wherein [Mo]wt %, [Nb]wt %, [Ta]wt % and [W]wt % are percentages of Mo, Nb, Ta and W, respectively, based on the weight of the composition.
The casting and the metal working suitable for use in the process of the present invention are not limited, and can be any known techniques in the art.
A typical quenching method used in the process of the present application is water quenching; however, any methods known in the art which have a cooling rate greater than 10xc2x0 C., preferably 20xc2x0 C., per second, can also be used.
The medical device prepared by the process of the present invention can be an orthopedic implant, a dental implant, a dental crown, a dental bridge or a denture framework.
Some of the preferred embodiments according to the present invention will be described in the following examples, that are intended as illustrative only since numerous modifications and variations therein will be apparent to those skilled in the art.